In the second step of the Vault process the backup images that are candidates to be transferred off site are duplicated. That is the image duplication step. Vault writes copies of the backup images on the media that you can eject and transfer off site.
Image duplication is optional. If you send your only backup image off site or create multiple original backup images and send one or more of those images off site, you do not have to duplicate images in Vault and therefore do not have to configure the duplication step. However, you must write the original image to media in the off-site volume pool so it is ejected and transferred off site.